Secret signaling



M .Vf u m3 0, w rin ...u m n s ,m a 5, .r n# 1 n e e u v/ mv@ 2 .l l l h6J 0 n\/u R 2 a m Q VJ n G 8. 6.---, D, :D m t. M F 0 L M E E E E Zw H nm w P a L m uz l /9 R F z \|l/ W n T Q S R .m W Pu I n 9 o l /lu z 0. 42. 1, .W 6 F 2 m m J Q (A Jan. 26 1926.

1,571,005 R. v. 1 HARTLEY SECRET SIGNALING Original Filed Sept- 8, 19202 Sheets-Sheet 2 lnvenror:

,Qa/,0k V. L. Hanf/ey Patented Jan. ze, 1926.

UNITED STATES PATENT OFFICE.

RALPH V. L. HARTLEY. OF SOUTH ORANGE, NEW JERSEY. ASSIGNOR TO WESTERNELECTRIC COMPANY, INCORPORATED, OF NEW YORK. N. Y.. A CORPORATION OF NEWYORK.

SECRET SIGNALING.

Application led September 8, 1920, Serial No. 408,890. Renewed November14, 1925.

To al? whom z't may concern.

Be it known that I, RALPH V. L. HAnTmir, a citfen of the United States,residing at South Orange, in the county of Essex, State of New Jersey,have invented certain new and useful Improvements in Secret Signaling,of which the following is a full, clear, concise, and exact description.

The present invention relates to secret transmission of electricalcurrents or waves for signaling or other purposes.

More specifically the invention relates to the type of wave transmissionin which a sustained wave is modulated or controlled vin accordance withthe wave to be secretly transmitted. A sustained wave used in thismanner is sometimes called a carrier wave, and generally has a constanthigh frequency. In the present invention, however, the carrier wave maybe audible and is made to have a cyclically varying frequency, the rateof variation being' very low in comparison with the frequency of thesignaling waves to be transmitted.

It is an object of the invention to enable audio frequency currentsrepresenting intelligence, but having a form which renders themunintelligible when received by ordinary means, to be` eiiicientlytransmitted to the practically complete exclusion of components whichare unnecessary and which might4 tend to lessen the difficulty ofobtaining` intelligible indications from the transmitted currents.

It is well known that when a carrier wave is modulated by a range of lowfrequency waves such as speech waves, a number of different frequencycomponents result, among which are an unmodulated component of theoriginal carrier frequency and two sidebands representing the sum anddifference, respectively, of the carrier frequency and the speechfrequencies. -If all of these components are transmitted over atelephone line. the speech frequency can be received by the use of asimple detector. If the unmodulated carrier frequenc1 component iscompletely suppressed and only one sideband is transmitted. it isimpossible to receive the speech by simple detection, but

if aiwave of the carrier frequency is combined at the receiver with thetransmitted sideband, adetector will yield the speech waves. A system inwhich only one sideband is transmitted, therefore, possesses aconsiderable degree of secrecy, since an outsider would have somedifficulty in discovering the frequency of the wave which it isnecessary to use at the detector in order to receive speech. It isconceivable, howe-ver, that one might hit upon the proper frequency bytrial, and when once this has been discovered there would be but littledifficulty in receiving the speech.

It has been proposed to use a continuously varying carrier frequency inorder to make it necessary to know not only the carrier frequency at anyinstant, but the manner in which it is varied before the transmittedmessage can be received. By varying, the

carrier frequency over a considerable range, 4

and especially by employing a more or less complex scheme of variationof the carrier frequency, a high degree of secrecy is obtainable.

If a wire transmission system is used, it. is desirable to suppresscompletely the currents of the original voice frequencies. since ifanyof these get to the line it might be possible to filter them out andamplify them suiiciently to render them receivable. It is also necessaryto suppress practically completely the unmodulated carrier frequencycomponent and one ofthe sidebands. It is desirable also to suppressdouble frcquency `components and the other frequency components whichare not to serve any useful purpose. If the lower sideband is the onewhich is to be transmitted. and if the lowest essential frequencyrepresented in it is above the highest voice frequency then a high passand a low pass filter in series in the output of the modulator can beused. The high `pass filter will suppress all currents of thefrequencies from zero up to the upper limit of the voice range. whilethe low pass filter will suppress all currents of frequencies higherthan the lower sideband including components of the carrier frequencyandthe harmonics. If the lower sideband, however, `contains frequenciesso low as to overlap the voice range, a filter cannot be used tosuppress the voice, but a balanced modulator may be used fer thispurpose as will be described hereinafter. In either case a low passfilter may be used to suppress the umnodulated carrier component, theupper sideband and the high harmonics. It will be shown hereinafter,however, that if a low pass filter having a lfiXed cutoff frequency isused to suppress the carrier it will permit of only a very limited rangeof Variation of the carrier frequency.

It is a further object of the present invention to overcome thislimitation and to make possible a variation of the carrier frequencyover any desired range while still preventing the unmodulated componentof carrier frequency and other undesired frequency components from beingtransmitted. In the present instance` this object is accomplished byvarying,vr simultaneously with the carrier frequency the frequency abovewhich all frequencies are suppressed. This may be done either by the useof a plurality of filters of successively higher'cutofi' frequencies -orby a filter having a variable cutoff.

In the drawings to which reference will now be made for a more detaileddescription of the invention` Figs. 1,A 2, 3 and 4 are schematic circuitdiagrams illustrating forms of embodiments which the invention may take`Fig. lf1 is a graphic representar tion of certain frequency relationsmade use of in describing the invention, Fig. 2a illustrates amodification of certain details of Fic. 2, and Fig. 5 shows a modifiedform of filter suitable for use in the system of the invention.

In the system to be described, it is assumed that the carrier frequency`varies cvcliffllv in any desired manner between the limits c and fv-I-dwhere d varies between r'ero nnd a predetermined limiting value. If thesneech frequencvband as a whole is 'renr^sented bv s and if the carrierwave is l nent and the upper sideband. This means that the cutofffrequency of the filter must always lie between the carrier .frequencyfv-l-d and the frequency in the band v-l-d-fs lying nearest to thecarrier frequency. The

permissible range ofe variation of the carrier frequency is limited..therefore, to something less than the lowest essential voice frequency.This is illustrated in Fig. la. The upper portion of the diagramrepresents the position of the speech modulated wave in terms offrequency relative to the cutoffl frequency F of the filter at theinstant when the carrier frequency has been varied to its upperpermissible limit (dzmax.) and the lower portion of the figurerepresents the conditions at the instant when the carrier frequency hasits lowest frequency value ((1:0). The carrier frequency is in thisfigure denoted by the heavy vertical line. The sidebands are denoted bys, the one to the left of the carrier 'frequency' being assumed to bethe lower and the one at the right the upper sideband. If the filter isso designed that all frequencies to the left of the cutoff frequency FVof the filter are transmitted while all frequencies to the right aresupthan that shown by max.. it will be seen` bearing in mind that thesidebands would be shifted with the carrier, that some of the lowersidebands would be thrown into the suppression range of the filter andthis would result in impairing the transmission of the speech. Ifthecarrier goes to a lower value than that indicated by (Lto, it willpass into the transmission range of the fil' ter. It is evidenttherefore that with a filter having a fixed cutoff. the maximum permisfsible range of variation of the carrier frequency issomewhat less thanthe frequency Iinterval between the carrier and the nearer edge of thesideband.

According to the present invention, the cutoff frequency F is changedwith the carrier so that to whatever extent the carrier frequencyshifts, the cutoff frequency changes in such a manner as to suppress thecarrier, but to permit the transmission of one of the sidebands. foraccomplishing this is shown in Fig. 1. In this figure a transmittingrarrangement is shown. IThe modulator M is supplied with carrieroscillations from the generator O and with speech frequencies frein thecircuit 2. The speech waves originate in the microphone circuit 3, whichis coupled to the circuit 2 through a low-pass filter 4. This filter isso designed as to transmit alll of the essential. voice frequencies, butto suppress the high harmonics of the voice so that essential speechfrequencies confined to a limited range are used to modulate thecarrier. The modulator M is of the balanced type comprising the twodischarge devices 5 and G connected so as to suppress transmission ofthe speech waves. The speech waves, it will be noted, are impressed Onearrangement upon the common connection between the filaments and thegrids through the coil 7, while the carrier wave from the oscillator Ois impressed in an opposite sense upon the two tubes by means of thecoupling 8. Due to the mode Aof connection employed here, the unmodifiedvoice frequency components are suppressed, but the unmodified carriercom onent will be transmitted. The carrier equency may be high incomparison with speech frequencies but it may also be of audiblefrequency. There will be no confusion of the original speech currentsand the side band currents for the reason that the former are nottransmitted by modulator M. c

A filter 9 is employed to suppress the carrier frequency, but to permittheI transmission of the lower sideband resulting from modulation of thecarrier by the speech frequency currents. The filter 9 is, asis also.the filter 4, of the general type described in the patent to Campbell,No. 1,227,113, dated May 22, 1917. but filter 9 is provided with meansfor making its upper limiting frequency variable at will. The filter 9comprises a number of. sections each of which includes inductiveelements in series with the line and capacitative elements in shunt ofthe line. The cutoff frequency of the filter depends upon the product ofthe` inductance and the capacity values, and hence by varying themagnitude of either the inductance or the capacity, it is possible tovary the cutoff frequency. In, the drawing, the capacity in shunt of theline consists of fixed capacity. elements 10 and variable capacityelements 11 in parallel .with each other. The variable capacity elementseach comprise a stationary plate and a rotatable plate which is arrangedoo be driven by the motor 12, and their plates or armatures may be sodesigned as to introduce any desired changes into the capacity of thesecondensers. In this way the cutoff value of the filter may be varied inany one of a number of ways by changing the shape and size of thecondensers even though the speed of rotation of the motor remains' thesame. As pointed out above, the carrier, frequency in this systemchanges cyclically between redetermined limits. For introducing t ese Ifrequency variations the oscillator O may be supplied with a cyclicallyvarying capacity element 13 mounted on the same shaft as the condensers11, and the system may be so controlled that as the carrier frequency,supplied from the oscillator O, in-

) creases or decreases, the cutoff value of the filter will follow thechan es in the carrier frequency by virtue of t e. variations incapacity of the condensers 11. With an arrangement of this kind it ispossible to 5 vary the carrier frequency in accordance with any desiredscheme of variation between relatively wide limits, and still permit thetransmission of the lower sideband, but suppress the transmission of theunmodulatcd component of the carrier frequency.

Since in order to receive the Wave. transmitted from the circuit in Fig.1, it is necessai-y to supply to the detector a wave of the carrierfrequency, it is necessary to provide at the receiver an oscillator, thefrequency of which varies in the same manner as that of the oscillatorO. This may be done conveniently by providing at the receiver a constantspeed motor similar to 12 and by synchronizing the'movements of thc twomotorsy in any well known manner.

For two-way secret communication. a rcceiver may be provided atthe samestation as the transmitter shown in Fig. 1, and this receiver may besupplied with the necessary carrier frequency wave from the oscillator Oprovided the same carrier frequenc is used for transmission in the twodirections. The type of receiver necessary will be clear, it isbelieved, from the description that has been given of the transmittingcircuit, but the circuit arrangement preferably used at the receiverwill be described more in detail in connection with Fig. 3.

The system of secret signaling according to the invention may be eithersimplex or multiplex. Fig. 2 shows a schematic layout of a terminalstation on the multiplex line 14, in which provision is made fortransmitting simultaneously two messages in each direction. By anextension of the arrangement illustrated in this fi ure 4it is evidentthat a larger number o channels maybe provided. The multiplex line 14has a balancing network 15 byvirtue of which in connection with thehybrid coil 16, 17, currents may be impressed on the line and receivedfrom the line without interfering with one another. Bridgedacross theline 14 is a transmitting loop S to which the transmitting circuits ofall of the channels are connected, and inductively related to the hybridcoils 16, 17 is a receiving loop R to which the receiving circuits ofthe channels are connected. In this system instead of showing atransmitter and receiver connected with the modulator and detector, lowfrequency lines v such as ordinary telephone lines L,l and L2 areconnected with the high frequency sets to enable two-way repeatingbetween the multiplex line and the telephone lines. Each channel issupplied with carrier frequency oscillations from the generators such asindicated at O1 and O2. The voice currents received over theline L1, forinstance, are impressed upon the modulator M1 in the same manner asindicated in Fig. 1, and the lll) carrier. wave from the, oscillator O14ismodulated in accordance with speech or other low frequency currentstransmitted overline L1. A transmitting filter SFl, which may be ofthetype shown in Fig. 1, suppresses transmission of' the unmodulatedcarrier frequency component, but permits transmission of the lowersideband from the modulator to the line through sending loop S. At thesame time speech currents received over the line L2 modulate a car-- maybe 4similar in every respect to that' shown in this figure, the natureof the receiving circuit may be understood by referenceito the receivingapparat-us at the terminal station shown. It will be assumed that wavesof the same carrier .frequency are used for transmitting in the twodirections and that the lower sideband is in each case transmitted,although if preferred, the upper sideband could-be transmitted( anddifferent carrie-r frequencies for. the two directions might be used.Modulated waves received from the line 14 inthe loop R and intended forthe low frequency line L1 are selectively received lby the receivingfilter RF from which they pass into the detector D,. As pointed outabove, it is necessary to supply to the detector a carrier wavecomponent of the same nature as that which was modulated at thedistant-'station by the message wave to be detected, but of which 'theunmodulated component was suppressed at the distant station. This -wavemay introduced to the detector D1 from the oscil# lator O1, as is shown.In the same manner modulating waves to be retransmitted over the line L2are selectively received'by thev receiving filter BF2, and' are detectedby the detector D2. The receiving' filters, the transf mittingfiltersand the oscillators may be controlled cyclically by the motor 12 in thesame manner as that indicated in Fig. 1. ,As stated above, a similarmotor and similar high frequency apparatus will be provided`` at thedistant'station of the line, andas it 1s necessary to maintainsynchronlsm between the shafts which drive the high fre-` quencyapparatus at the two stations, any suitable method of synchronizingmaybe employed.` For urposes of illustration,- however, an impu sesender 18 is shown combe,v transmitted b v the other channels.'v

`contact successively in its rotation with the commutator .segments 19.These contacts close a circult from battery 20 through the low-passfilter LP, thel sending` loop S and the main line 14. The low-passfilter LP prevents waves-of carrier. frequency from nding access to thecommutator circuit, but permits Athe transmission of periodic impulsesto the main line 14. The commutator may be arranged to transmit anynumber of these impulses per revolution of the motor, four such impulsesbeing transmitted in the arrangement shown. At the distant station theseimpulses are received through a filter similar to LP and may be used 1n,accordance with any one of a number of lwell known synchronizing systemsfor introducing phase corrections into the motion'of a synchronously,driven shaft which may con' trol the oscillators and. the filters atthat station. It should be noted that these impulses bear no Idefiniterelation to the carrier frequencies or to the scheme of variationv ofthe carriers, since any desired number of correcting impulses maybe'transmtted per revolution, and-'the condensers which control theoscillators and the filters mayl be given any desired shape and may bechanged from time to time. It isl evident, therefore that the correctingimpulses will give no clue to the'conditions that it is necessary toknow in order to receive the message, and that the correct-ing impulsesdo'not need to be concealed.

The filter that is preferably used in the circuits thus far-shown, is 'alow-pass filter which'transmits vfreely currents of all fre'- quenciesbetween zeroI anda predetermined upper limit.` If the systemis .used ona multiplex-line, as 'indicated in Fig. 2, the filters of 'the differentchannels will differ as to their cutoff frequencies and the filterhaving the highest cutoff frequency of any of the channels will admitthe frequencies To prevent thehpossibilityQof lcross-talk, band filtersBF may be'inserte'd between the variable filtersv and the line. .Thesefilters; may be fixed filters of suitable constants to permit thepassage ofafllfrequenciesthatf -it is desired to. ganSmit-{through thechannel jto which they are connected, but toysuppress ltransmissionof"frequencies4 used "by .each of the other channels.l l I It has' beenassumed that the cutoE value ofthe filtrsthat have been described isvaried by 1v1/trying the shunt 'capacity only. For most 1purposes thi-swill probably be sufficient. owever, the impedance of a filter whosecutoff frequency isvaried in this manner, would also vary to aconsiderable extent. Since the expression for both the cutoff frequencyandthe impedance'involve the inductance and the capacity of the filtersections, it may be dsirable in some cases to vary cyclically theinductance as well as the capacity in order to maintain the impedancemore nearly constant. A filter in which provision is made for doing thisis indicated in Fig. 5, which in view of the other figures will beunderstood without further explanation.

It is, of course, possible to operate one or two channels of themultiple system of Fig. 2 with an audio frequency carrier wave. Thefirst channel, might, for example, use the lower side band of amodulate-d carrier wave of 2,500 cycles frequency and the secondchannel, the lower side' band of a modulated carrier wave of 5,000cycles frequency.

Fig. 2 illustrates a modification of Fig. 2 in which a variable bandfilter having variable series and shunt capacities may be used toreplace a pair of lters such as SF1 and BF or RF1 and BF in each one-waychannel. The characteristic of the filter of Fig. 2a is such that theWidth of its transmission band may remain approximately constant as itsupper and lower cut-0E frequencies simultaneously increase or decrease.This type of filter is accordingly well adapted to transmit a modulationside band of fixed range, but of varying carrier frequency.

Instead of varying the cutoff' frequency of the lters which are used tosuppress the carrier frequency, arrangements are shown in Figs. 3 and 4for using a plurality of fixed lters having successively differentcutoff frequencies.

In Fig. 3, the modulator M is supplied by the carrier wave from theoscillator O and by speech waves from the circuit 22, as

vin the 4previous figures. The output of the modulator leads to rotatingswitch arms 23 arranged to connect the leads 24 of the modulatorsuccessively to the input terminals of the respective filters SF, SF andSF. The output sides of these filters are connected in parallel to theoutgoing line 25. The switch arms 23 may be driven from the shaft of themotor` 12, which also drives the variable condenser 13 for introducingvariations into the oscillator frequency. If the cutoff frequency of thesuccessive filters differs by less than the frequency interval betweenthe carrier and the near edge of the side band to be transmitted, it isevident that as the carrier frequency rises to the point where the lowersideband is about to pass into the suppression range of the filter SF,the carrier frequency has exceeded the cutoff value of the filter SF.The filter SF can, therefore, be switched out of the circuit and thefilter SF can be thrown into the circuit and the carrier frequency canthen be permitted to increase to such a hi h value that the lowersideband is about to is cut-off by this filter when the third filter SFmay be switched in. This process may be continued indefinitely byproviding enough filters. The switch arms 23 are provided withsufiiciently broad contacts to enable the next succeeding filter to beincluded in the circuit before the one previously used is disconnectedso that no discontinuity in the transmission occurs. As the carrierfrequency decreases in value, the filters will be switched into thecircuit in the reverse order and any suitable means of controlling theswitch arms 23 by the continuously rotating shaft of the motor 12 may beemployed. It is considered unnecessary to show further details of suchan arrangement.

Referring now to the receiving circuit of Fig. 3, modulated incomingwaves in the circuit 26 are impressedupon the bank of receiving filtersRF, RF and RF. Switch arms 27 connected to the shaft of the motor 12 andoperating in the same manner as switch arms 23, cut the successivereceiving filters into and out of the line leading from circuit 26 tothe detector I). In the case of a simplex systemit probably would not benecessary to use a plurality of receiving filters, provided arrangementswere used for venting transmission of ythe carrier Wave back to theline. However, the three receiving filters have been indicated forpurposes of illustration, since in a multiplex system it might bedesired to have the frequencies of the currents of all the channels varytogether in the same sense, so that at certain instants the channelwould be using currents of certain frequencies which at other instantsmight be used by a different channel, in which case it would benecessary to provide receiving filters corresponding to the transmittingfilters. The detector D may be of the type used in any of the circuitsheretofore shown. It comprises two discharge tubes 28 and 29 connectedin balanced relation, the received sideband being impressed upon thetvwo vgrid circuits through the coupling 30. The necessary unmodulatedcomponent of carrier Wave frequency might be applied either to the inputcircuit or to the output circuit. It is preferably applied to the outputor plate circuit, as is indicated by theconnection 31, since suchconnection prevents transmission of the carrier frequency back to theline 26 by virture of the unilateral conductivit of the dischargedevices 28- and 29. By ta ing 4carrier frequency and the cut-offfrequency of the transmitting filters is indicated in Fig. 4. In thisfigure the d ottedl line 33 indicates the shaft of the motor such asmotor- 12 in the other figures.'y This shaft merely serves to drive thecommutator 34 which is shown provided with conducting and insulatingsegments irregular in character. The

, shape and disposition of these segments de- 4 output of the modulatorM through thev filter SF. This circuit may be traced from the lead 38 ofthe modulator to the contact spring 39, relay armature'40 and lead 41 toone terminal of the filter SF and from the other terminal of the filterthrough the lead 42 to the opposite lead 43 of the modulator M. Relayarmature 40 in its retracted position permits the closure of contacts 44which close a shunt around a portion of the inductance 45 whichinductance, together with the condenser shown, controls the period ofthe oscillator O furnishing carrier waves to the modulator M. Theadjustment of the oscillator and the constants of the filter SF areVarranged so that when the shunting contacts 44 are closed, the filter SFsuppresses the unmodulated component of the carrier frequency andcomponents of higher frequency "but transmits -the lower sideband. Whenthe rela 37 is energized by closure of the circuit t rough a conductingsegment of the commutator, its armatures are attracted and armature 4()shifts the circuit from the contact spring 39 to the spring 46,'

and immediately thereafter breaks the con- -5 tact between the spring 39and the armature 40` At the same time it separates the contacts 44,thereby removing the shunt connection from the inductance 45. The out#put of the modulator is now connected through conductor 38, contactsprings 39 andV 46, conductor 47 to one terminal of the filter' SF andfrom 'the other terminal of the filter to the opposite lead 43 of themodulator. The filter SF is so designed that when the shunt throughcontacts 44 `is broken and the corresponding variation is introducedinto the frequency of the oscil` lator, this filter will suppress theunmodu lated carrier 4frequency component and components of higherfrequencies but will per' mit the transmission of the lower sideband.

l,By alternately making and breakin the'circuit of relay 37, therefore,a sud en shift 1n the carrler frequency is produced and. the filters arealternately controlled to suppress the unmodulated carrier frequencycompo-l nentand, to transmit the lower sideban'd. Relay 37 is also shownas provided with an armature 48, and by a set of contacts and circuitssimilar to those controlled by armature 40, causesthe receiving filtersRF and l RF tobe alternately included between the incoming circuit 49and the detector D. As noted above, it may not in all cases be necessaryor desirable to provide a plurality 'of receiving filters, but 1n somecases a single filter will suffice.

The circuit arrangements that have been illustrated and described areintended merely as representative of preferred embodiments of theinvention, and it is obvious. that many other arrangements' are'possible' and will occur to those skilled' in the art. The invention,therefore, is not to be construed Ias limited to the specific circuitarrangements that have beeny disclosed, but only by the scope of theappended claims.

What is claimed is:

1. In asignaling system, a source ofI car- Y rier waves of continuouslyvarying frequency, means to modulate said waves in accordance withsignals to produce upper and lower side bands of frequency components,selective circuits for selectively transmitting a'portion'of saidfrequency components to the practical exclusion of all of the componentsin one of said side bands, and means coordinated with the'said source ofcarrier waves for varying the selective range of transmission of saidselective circuits in step with the variations in the carrier frequencyto cause said selective circuits to suppress substantially the whole ofthe same side band at all frequencies of the carrier wave.

2. In a signalingsystem, a source of carrier waves of continuouslyVvarying frequency, means to modulate said wave linl accordance withsignals to produce upper and lower side bands of frequency components,selective circuits for selectively transmittinga portion. of saidfrequency components to the practical exclusion of the unmodulatedcarrier frequency component and all of the components in one of saidside bands, and means coordinated with said source of carrier waves forvarying the selective range of transmission of said selective circuitsin stepwith the variations in the carrier frequency to cause saidselective circuits to suppress said carrier frequency and substantiallythe whole of the same side bandcomponents at all frequencies ,of thecarrier wave.

3. In a signaling system a source of carrier waves of cychcally varyingfrequency, a modulator for modulating sald waves'in accordance withsignals to be transmitted, filtering circuits for suppressing theunmodulated carrier frequency component while permitting transmission ofone of the sidebands, and means for controlling said circuits to varythe limiting frequency between the transmitted and the suppressed rangesas the carrier frequency is varied.

4. A signaling system comprising transmitting and receiving stations,synchronously driven control members at said stations, means forproducing a complex wave at a transmitting station, selective circuitsat each station for selectively transmitting a portion of said wave andmeans under control of said synchronously driven' control members forcyclically varying the frequencies of said wave and for varying thetransmission characteristics of said selective circuits to transmit thecorresponding portion of said complex wave at its different frequencies.

5. In a modulated wave transmission system including a wave transmittingmedium, circuits for producing'for transmission a modulated carrierwave, a plurality of ltering circuits arranged for connection betweensaid wave producing circuits and the medium, having different cut-ofi:lfrequencies, means for changing the carrier frequency and forsimultaneously changing the connections of said filtering circuitsbetween said wave producing circuits and the transmission medium totransmit substantially 2 the same relative components of said modulatedwave and to suppress substantially the same relative components of saidmodulated wave after the carrier frequency is changed as before.

6. The method of modulated carrier wave signaling in which the carrierwave in cyelically varied in frequency, which method comprisesmodulating the carrier wave of varying frequency in accordance withsignals to produce side bands, producing cychc variations in the carrierfrequency of greater extent than one half the frequency intervalseparating said side bands, filtering the modulated waves to suppressthe carrier and one of the side bands while freely transmitting theother, and shifting the suppression range of the filtering circuitssimultaneously with the cyclical changes in the carrier frequency toenable separation between the carrier and side band to be suppressed.and the other side band at all the values of the carrier frequenc 7. nmodulated carrier wave transmission of signals comprising a band offrequencies lying between two finite frequencies neither of which iszero, the method of selecting one side band to the exclusion of thecarrier component and the other side band notwithstanding variations inthe carrier frequency of greater extent than that represented betweenzero and the lower limiting frequency of the signals to be transmitted.which method comprises filtering the modulated wave to separate one sideband from the carrier component and the other side band, and varying thecut-oil'l frequency of the filtering circuits in synchronism with thevariations in carrier fre uency to cause the cut-off frequency of theltering circuits to be within the frequency interval separating thecarrier frequency and the side band to be transmitted at all values ofthe carrier frequency.

8. The method of wave transmission, which comprises modulating a varyingfrequency carrier wave by a modulating wave to produce a modulated wave,suppressing from the resulting modulated wave com- -ponents of a rangeof frequencies including the original carrler frequency, and causing theabsolute value of the limiting frequency of the suppressed range to varyin accordance with the variation in the frequency of the carrier wave,whereby suppression of the said range of frequencies including theoriginal carrier frequency may take place Wit a greaterrange ofvariation of the carrier frequency than -would be possible with a fixedlimiting frequency.

9. A transmission system comprising means for producing a carrier waveof varying audio frequency,- modulating circuits for modulating saidwave in accordance with audio frequency currents to produce# resultantmodulated waves of frequencies overlapping the range occupied by saidaudio frequency modulatin waves, and means included in said modulatingcircuits for suppressing transmission of unmodified audio.

frequency modulating currents, while permitting transmission of said,modulated waves. y

10. Ina secret signaling system, a line, a source of audio frequencycarrier waves, a balanced modulator includin input and output individualbranches an a neutral in- `'put branch, a source of speech currentwavesa said source of speech current waves being associated with saidneutral branch, said source of carrier waves being differentiallyassociated with both input individual branches, said lines beingassociated differentially with both output individual branches, wherebythe speech waves are prevented from being transmitted to line, saidcarrier waves being of such low frequency that the resulting modulatedwaves overla in frequency the range occu ied by sai speech waves, and alow pass lter connected between said modulator and said line forselectively7 transmitting the lower side band from said modulatingcircuit, said filter suppressing transmission of the carrier wavecomponent.

11. In a signaling system, a line, modulating circuits for producing aspeech modulated carrier Wave having frequencies overlapping the rangeoccupied by the unmodied speech waves, said modulating circuitsincluding means to suppress transmission to the line ofthe unmodifiedspeech waves, and a low pass filter connected between said modulatingcircuits and said line for suppressing transmission of currents of thecarrier Wave frequency and of hlgher frequen- 10 cies.

In witness whereof, I hereunto subscribe my 'name this 1st day ofSeptember A. D.,

RALPH V. L. HARTLEY.

